1. Field of Invention
This invention relates to improved methods and apparatus for dynamic correction and minimization of aberrations produced in the beam of electrons or other charged particle in tubes or columns of the electron beam type. The invention is particularly well suited for use with electron beam tubes of the fly's eye compound lens type employing a two stage eight-fold coarse electrostatic deflector system and an array fine deflector system.
2. Prior Art Problem
U.S. Pat. No. 4,142,132, issued Feb. 27, 1979, entitled "Method and Means for Dynamic Correction of Electrostatic Deflector for Electron Beam Tube"--Kenneth J. Harte, inventor, describes and claims a greatly improved eight-fold electrostatic deflection system for electron beam and other charged particle beam tubes employing electrostatic deflection systems. The tube described in U.S. Pat. No. 4,142,132 is designed for use in an electron beam addressable memory wherein the number of data storage sites that the electron optical system can resolve at the target plane of the tube (at fixed current density), or the current density that can be achieved with such a tube (with a fixed number of data bit sites), varies inversely with the electron beam spot aberration at the target plane. As stated in the above referenced U.S. Pat. No. 4,142,132, electron beam spot aberration is introduced by an electrostatic deflector system as it causes the electron or other charged particle beam to traverse from a center-axis position across the x-y plane of a target surface to a particular x-y address bit site location whose x-y coordinates identify the data to be stored and/or retrieved. For maximum data storage capability on a given target surface area, electron beam spot aberration must be kept to a minimum.
The eight-fold electrostatic deflection system and methods of correction described in the above referenced U.S. Pat. No. 4,142,132 provide greatly improved performance and minimize to a considerable extent beam spot aberration at the target plane. The present invention is designed to complement the desirable features of the eight-fold deflection system and method of correction described in U.S. Pat. No. 4,142,132 and to thereby provide improved performance and minimization of beam spot aberration at the target plane.